Thermionic emission

Thermionic emission (including thermionic effect, Edison effect or Richardson - effect) is the emission of electrons from a heated hot cathode (typically in vacuo ). The minimum temperatures are above 900 K and strongly depend on the material of the surface.

General

The electrons to overcome the work function of the metal or the characteristic of the oxide film due to their thermal energy. If the free electrons are not sucked by an electric field, they form the hot cathode in the vacuum of a space charge cloud and invite any nearby electrodes against the " cathode" negative on. This effect can be used for the direct conversion of thermal energy into electrical energy. However, the efficiency of this thermionic generator is low.

For technical applications, the aim is to maintain the required temperature of the hot cathode as low as possible, by using materials with low work. This led to the development of the oxide cathode.

History

The effect was first described in 1873 by Frederick Guthrie. He discovered that a positively charged electroscope is discharged when they brought a grounded, glowing piece of metal in the vicinity. When a negatively charged electroscope nothing happens, resulting followed that glowing metal can only leave negative charge. Thomas Edison has rediscovered this phenomenon in 1880 in experiments with incandescent and reported 1883 based thereon application for a patent. Julius Elster and Hans Friedrich Geitel examined 1882-1889 systematically from a hot wire charge output. The saturation current density was recorded in 1901 by Owen Willans Richardson constructed in the Richardson equation, for which he was awarded the 1928 Nobel Prize in Physics.

Richardson equation

Richardson 's equation describes the current density J of the emerging of a metal at high temperature electrons. It reads

Here, T is the absolute temperature, the release We work function for electrons, kB is the Boltzmann constant and A is the Richardson constant.

The release work function for electrons is generally about 1 to 6 eV. The Richardson constant depends mainly on the used metal and the surface condition and is in short supply. For metal oxides, it is much lower.

After Saul Dushman (1883-1954), the Richardson constant can be estimated as follows:

Is also referred to as the Richardson - Dushman equation.

A correction term for the work function resulting in very high field strength by the Schottky effect. In this area it is called Schottky emission.

Applications

The thermionic emission is used to generate free electrons in the electron tubes. This in a high-vacuum vessel between the directly or indirectly heated thermionic cathode and anode flows (electron ) current which can be controlled, if necessary, by intervening grid. Electron tubes allow the amplification of electric signals in the audio frequency range and in the high frequency range, in transmitters and receivers. With electron tubes, it was possible not only to transmit Morse code, but also language, music and pictures.

The cathode ray tube ( cathode ray tube ) consists of an electron source with subsequent deflection. applications:

• To the electron beam melting and electron beam and electron beam welding, also in the scanning electron microscope.
• With fluorescent screen as a picture tube in old televisions and oscilloscopes.

Fluorescent hot cathode also use thermionic emission. Many other gas discharge lamps and carbon arc lamps, the electrodes by the discharge of heat is also applicable to thermionic emission is involved. Not the case, however, this is the case of cold -cathode tubes, fluorescent tubes, neon lamps and flash tubes.

Thermionic emission is still used in thyratrons, magnetrons, klystrons, traveling wave tubes and vacuum fluorescent displays. Again, it is used to generate free electrons.

By means of thermionic emission, the work function can be determined. By the electric field, which is required to remove the electrons from the cathode, but this is influenced, so that it is necessary to extrapolate the measured current on the field strength, E = 0.

Adverse effects of thermionic emission

Thermionic emission is in control grids of electron tubes ( ie when the grid is glowing due to heating ), however undesirable, here it leads to the so-called grid emission and hindering grid current, which can shift the operating point. Power tubes usually receive about cooling fins ( radiation cooled) at the ends of the grid carrier rods; latter are for good heat conduction usually made of copper.

In the thermionic emission switches between the switch contacts to the undesirable switching arc, which must be deleted to avoid damage by appropriate technical means leads.

Related effects

• Outside photo effect
• Field emission